J. A. Forrest

The glass transition in thin polymer films

Abstract In this article, we present a detailed account of important recent developments in the rapidly evolving area of glass transitions in thin polymer films. We review the case of polymer films supported by substrates, and show that a definite experimental consensus exists. We consider recent results from experimental studies of free-standing films of polystyrene. These studies have provided a thorough quantification of the behavior of glass transition anomalies in free-standing polymer films, and have served to motivate recent attempts at theoretical descriptions. We introduce and examine models, which have been proposed to explain the experimental observations and discuss the significance of these models.

Measuring the Surface Dynamics of Glassy Polymers

The motion of polymer chain segments cooled below the glass transition temperature slows markedly; with sufficient cooling, segmental motion becomes completely arrested. There is debate as to whether the chain segments near the free surface, or in thin films, are affected in the same way as the bulk material. By partially embedding and then removing gold nanospheres, we produced a high surface coverage of well-defined nanodeformations on a polystyrene surface; to probe the surface dynamics, we measured the time-dependent relaxation of these surface deformations as a function of temperature from 277 to 369 kelvin. Surface relaxation was observed at all temperatures, providing strong direct evidence for enhanced surface mobility relative to the bulk. The deviation from bulk α relaxation became more pronounced as the temperature was decreased below the bulk glass transition temperature. The temperature dependence of the relaxation time was much weaker than that of the bulk α relaxation of polystyrene, and the process exhibited no discernible temperature dependence between 277 and 307 kelvin.

A Direct Quantitative Measure of Surface Mobility in a Glassy Polymer

Polymer Film Behavior An ongoing debate in the understanding of the behavior of thin-film glassy polymers is whether there is nanoconfinement of large molecules or enhanced mobility near a free surface. Chai et al. (p. 994; see the Perspective by Chen et al.) prepared polymer films with a sharp step in the profile by depositing broken film fragments onto a uniform underlay. Atomic force microscopy revealed changes to the overall film profile with time at various temperatures. A transition was observed from localized motions to relaxation of the entire film at a temperature close to that of the bulk glass transition temperature. A step geometry is used to probe the behavior of a glassy polymer above and below its glass transition temperature. [Also see Perspective by Chen et al.] Thin polymer films have striking dynamical properties that differ from their bulk counterparts. With the simple geometry of a stepped polymer film on a substrate, we probe mobility above and below the glass transition temperature Tg. Above Tg the entire film flows, whereas below Tg only the near-surface region responds to the excess interfacial energy. An analytical thin-film model for flow limited to the free surface region shows excellent agreement with sub-Tg data. The system transitions from whole-film flow to surface localized flow over a narrow temperature region near the bulk Tg. The experiments and model provide a measure of surface mobility in a simple geometry where confinement and substrate effects are negligible. This fine control of the glassy rheology is of key interest to nanolithography among numerous other applications.

Size-dependent denaturing kinetics of bovine serum albumin adsorbed onto gold nanospheres

We have used localized surface plasmon resonance (LSPR) to monitor the kinetics of thermal denaturing of bovine serum albumin (BSA) adsorbed onto gold nanospheres of size 5 nm-100 nm. The effect of the protein on the LSPR was monitored by visible extinction spectroscopy. The wavelength of the peak extinction (resonance) is affected by the conformation of the adsorbed protein layer, and as such can be used as a very sensitive probe of thermal denaturing that is specific to the adsorbed (as opposed to free) protein. The time dependence of the denaturing is measured in the temperature range 60 °C–70 °C, and the lifetimes are used to calculate an activation barrier for thermal denaturing. The results show that thermally activated denaturing of proteins adsorbed onto nanoparticles has a nanoparticle-size-dependent activation barrier, and this barrier increases for decreasing particle size. This may have important implications for other protein-nanoparticle interactions.

PHASE SEPARATION MORPHOLOGY OF SPIN-COATED POLYMER BLEND THIN FILMS

We present the results of a study of the morphology of phase separation in thin films of two different polymer blend systems: polystyrene/polyisoprene and polystyrene/poly(methyl methacrylate). For each blend system, the two polymer components are dissolved in a common solvent. Spin coating of the ternary solutions (polymer blend/solvent) is used to confine the blends to a thin film geometry and to produce phase separation because of rapid evaporation of the solvent (solvent quench). As a quantitative measure of the phase separation morphology the average domain area of the minority component is measured as a function of the polystyrene mass fraction. For both blend systems we identify a small range of composition corresponding to a large increase in the average domain area. We show that the strong dependence of the average domain area on spin speed allows control over the quench time of the polymer blend thin films.

Imaging protein deposits on contact lens materials.

Purpose. The majority of studies investigating protein deposition on contact lens materials require that the deposit of interest be removed, potentially resulting in erroneous results if some proteins are not removed adequately. The purpose of this study was to investigate the use of in situ imaging methods to examine protein deposition on conventional poly(2-hydroxyethyl methacrylate) (polyHEMA)-based and silicone hydrogel contact lens materials. Methods. Six silicone hydrogel and five polyHEMA-based hydrogel contact lens materials were examined by Atomic Force Microscopy (AFM) and/or Scanning Electron Microscopy (SEM) techniques, after being deposited with proteins in an in vitro model. AFM studies examined lenses deposited solely with lysozyme at approximate physiological concentrations and SEM studies were conducted on lenses exposed to a dilute mixture of lysozyme and albumin-conjugated gold spheres. Results. AFM studies demonstrated that the lens materials had markedly differing surface topographies. SEM results showed that galyfilcon A and balafilcon A lenses deposited both lysozyme and albumin in relatively large aggregates, as compared with lotrafilcon A and B, in which the proteins were deposited in a more evenly spread, monolayer formation. Polymacon lenses deposited more protein than any of the silicone hydrogel materials and much of the protein was aggregated together. AFM data indicated that balafilcon A, lotrafilcon A and polymacon deposited lysozyme in a similar manner, with very little lysozyme being deposited in discrete areas. Galyfilcon A behaved very differently, with the lysozyme exhibiting both aggregates as well as string-like formations over the lens surface. Conclusions. Imaging techniques that allow proteins to be examined in situ show much promise for determining the extent and physical characterization of protein on contact lens materials. These techniques indicate that the pattern of deposition of proteins onto silicone hydrogel contact lens materials differs between materials, depending upon their bulk and surface composition.

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from Tg + 8 K to Tg − 83 K and times as long as 105 minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk Tg value. The surface relaxation times are only weakly temperature dependent and near Tg, the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.

Effect of atmosphere on reductions in the glass transition of thin polystyrene films

We have used nulling ellipsometry to measure the glass transition temperature, Tg , of thin films of polystyrene in ambient, dry nitrogen, and vacuum environments. For all environments, the measured Tg values decrease with decreasing film thickness in a way that is quantitatively similar to previously reported studies in ambient conditions. These results provide strong reinforcement of previous conclusions that such reduced Tg values are an intrinsic property of the confined material. Furthermore, the results are in contrast to recent reports which suggest that the Tg reductions measured by many researchers are the results of artifacts (i.e. degradation of the polymer due to annealing in ambient conditions, or moisture content).

Phase separation morphology of thin films of polystyrene/polyisoprene blends

We present the results of a study of the morphology of phase separation in a thin film blend of polystyrene (PS) and polyisoprene (PI) in a common solvent of toluene. The blend is quenched by rapid solvent evaporation using a spincoating technique rather than a temperature quench. The mass fraction of polystyrene is varied to determine the effect of the substrate on thin film phase separation morphology. We compare the phase separation morphology for very thin films of the PS/PI blend cast onto three different substrates: Si(001) with a native oxide layer (Si (SINGLEBOND) SiOx), Si(001) etched in hydrofluoric acid (Si-H), and a Au/Pd alloy sputtered onto Si(001). We observe large differences between the morphologies of 1000 A thick blend films on the Si(SINGLEBOND) SiOx and Si-H substrates as the mass fraction is varied due to the difference in the wetting properties of PS on the two substrates. Smaller differences are observed between the films on the Si(SINGLEBOND) SiOx and Au/Pd substrates only for film thicknesses h < 600 A.

Quartz crystal microbalance study of protein adsorption kinetics on poly(2-hydroxyethyl methacrylate).

The interaction of macromolecules with artificial biomaterials may lead to potentially serious complications upon implantation into a biological environment. The interaction of one of the most widely used biomaterials, polyHEMA, with lysozyme, bovine serum albumin (BSA), and lactoferrin was investigated using quartz crystal microbalance (QCM). The concentration dependence of adsorption was measured for the aforementioned proteins individually as well as for lysozyme-BSA, and lysozyme-lactoferrin combinations. An extension of Voinovas viscoelastic model to n layers was used to create thickness-time graphs for adsorption. For each of lactoferrin and lysozyme, two distinctly different timescales of adsorption could be differentiated. However, the mechanisms of adsorption appeared to differ between the two. Negative dissipation shifts were measured for low concentrations of lysozyme, trending to positive dissipation at higher concentrations. This suggested that lysozyme was adsorbed initially into the matrix, stiffening the hydrogel, and later onto the surface of polyHEMA. Additionally, trials with commercial no-rub cleaning solutions indicated little added effectiveness over buffer solutions. Mixtures of proteins showed behaviour which differed in some cases from the simple combination of single protein adsorption experiments.